Incremental Rendering of Deformable Trimmed NURBS Surfaces

Incremental Rendering of Deformable Trimmed NURBS Surfaces
Incremental Rendering of
Deformable Trimmed NURBS Surfaces
Gary K. L. Cheung†
[email protected]
†
Rynson W.H. Lau† ‡
[email protected]
Department of Computer Engineering & Information Technology, City University of Hong Kong, Hong Kong
‡
Department of Computer Science, City University of Hong Kong, Hong Kong
ABSTRACT
produce a variety of shapes simply by manipulating their
control points and weights. However, NURBS surfaces are
seldom used in interactive applications that demand realtime rendering performance because of their high rendering
cost. There have been a lot of work carried out to address
this problem. Most of the methods developed are based on
tessellation [1, 6, 8, 9, 10, 17]. This tessellation process
subdivides the NURBS surfaces into polygons so that the
hardware graphics accelerator, if present, may render the
polygons in real-time. However, this process is computationally very expensive. As a NURBS surface is deforming,
this process must be executed in each frame to reflect the
change of the object shape. Since in many real-time applications such as computer games, we may want to have
many deformable objects in the environment. Existing rendering methods would be difficult to render these objects in
real-time.
Trimmed NURBS surfaces are often used to model smooth
and complex objects. Unfortunately, most existing hardware graphics accelerators cannot render them directly. Although there are a lot of methods proposed to accelerate
the rendering of such surfaces, majority of them are based
on tessellation, which is developed primarily for handling
non-deforming objects. For an object that may deform in
run-time, such as clothing, facial expression, human and animal character, the tessellation process will need to be performed repeatedly while the object is deforming. However,
as the tessellation process is very time consuming, interactive display of deforming objects is difficult. This explains
why deformable objects are rarely used in virtual reality
applications. In this paper, we present a efficient method
for incremental rendering of deformable trimmed NURBS
surfaces. This method can handle both trimmed surface deformation and trimming curve deformation. Experimental
results show that our method performs significantly faster
than the method used in OpenGL.
Earlier, we proposed an efficient method for rendering deforming NURBS surfaces [13]. The method pre-computes
a polygon model and a set of deformation coefficients for
each deformable NURBS surface. During run-time, it incrementally updates the pre-computed polygon model of each
deforming surface and progressively refines the resolution of
the model according to the change in the surface curvature.
We have shown that this method is much more efficient than
existing methods. Recently, we have applied this method to
develop a distributed virtual sculpting system [14, 15]. We
have also extended the method to cover various types of deformable parametric free-form surfaces [12]. However, we
have learnt from our experience that in order to represent
real objects, such as human faces with eyes and mouths,
many NURBS surfaces need to be used. This is because our
method can only support regular NURBS surfaces. To represent an object with holes, we need to combine many regular NURBS surfaces to model a single object. To overcome
this limitation, our objective of this project is to extend the
method to support trimming [5], which is a technique to
allow arbitrary regions of a NURBS surface to be cut out,
resulting in a non-regular NURBS surface.
Categories and Subject Descriptors
I.3.5 [Computational Geometry and Object Modeling]: Curve, surface, solid, and object representations; I.3.5
[Methodology and Techniques]: Interaction techniques
Keywords
Deformable objects, NURBS surfaces, trimmed surfaces, realtime rendering
1.
Frederick W.B. Li†
[email protected]
INTRODUCTION
Deformable objects have been considered as important to
virtual reality applications, as they may model clothing, facial expression, human and animal characters. In particular,
Non-Uniform Rational B-Splines (NURBS) [4, 16] are often
employed to represent such objects as they can be used to
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In this paper, we describe a method to extend our NURBS
rendering method [13] to support trimming. The rest of the
paper is outlined as follows. Section 2 provides a brief survey on related work. Section 3 describes how we handle the
deformable trimmed NURBS surfaces. Section 4 presents
our method for tessellating trimmed NURBS surfaces. Sec-
48
method, like most of the other methods, is not suitable for
interactive rendering of trimmed NURBS surfaces.
tion 5 describes how to incremental update trimming curves
and trimmed NURBS surfaces as they deform. Section 6
demonstrates the performance of the method through some
experiments. Finally, section 7 briefly concludes the paper.
2.
3. HANDLING OF DEFORMABLE
TRIMMED NURBS SURFACES
RELATED WORK
In our earlier work, we developed a technique for efficient
rendering of deformable NURBS surfaces [11, 13]. The basic idea of this method is to maintain two data structures of
each surface, the surface model and a polygon model representing the surface model. As the surface deforms, the polygon model is not regenerated through tessellation. Instead,
it is incrementally updated to represent the deforming surface. To handle trimming curves efficiently, we also model
them in a similar way.
Efficient rendering of trimmed NURBS surfaces has been a
challenging research area for decades. There are many methods proposed for tessellating trimmed NURBS surfaces into
polygons for rendering. In particular, Shantz et al. [18] propose an adaptive forward differencing technique to evaluate
the points on the surface incrementally to produce a polygon model for rendering. As the method may adjust the
forward differencing step adaptively, it could optimize the
number of surface points generated according to the surface
curvature or other approximation criteria. Rockwood et al.
[17] propose an alternative method to accelerate the tessellation process. It first converts the surface to Bézier patches
with knot insertion. For those trimmed patches, it further
subdivides them into a list of uv-monotone patches. Each
patch is then tessellated into polygons by the coving and
tiling process. A variant of this method has been implemented in the OpenGL library.
3.1 Handling of the NURBS Surfaces
The polygon model of a surface can be obtained by evaluating the surface equation with some discrete parametric
values. If a control point is moved from Pi,j to P i,j with a
→
−
displacement vector V = P i,j − Pi,j , the incremental difference between the two polygon models of the surface before
and after the control point movement is:
→
−
(P i,j − Pi,j )wi,j Ni,p (u)Nj,q (v)
= αi,j V
S(u, v) − S(u, v) = m n
w
N
(u)N
(v)
t,q
s=0
t=0 s,t s,p
(1)
where S(u, v) and S(u, v) are the polygon models of the
surface before and after the control point movement, respectively. αi,j is called the deformation coefficient defined
as follows:
Abi-Ezzi et al. [1] tessellate trimmed NURBS surfaces in
a way similar to [17], but it further minimizes the number
of patches needed to be tessellated by culling out the invisible patches dynamically during run-time. Kumar et al.
[9] improve the performance of the tessellation process by
avoiding the operation of subdividing Bézier patches into uvmonotone patches. Instead, it directly tessellates the Bézier
patches into polygons for rendering. However, to allow fast
back-patch culling, it needs to compute pseudo normal surfaces for all Bézier patches. In [10], they enhanced their
method in [9] by constructing super-surfaces on the Bézier
patches to allow a further reduction in the number of polygons of the resulting polygon model.
wi,j Ni,p (u)Nj,q (v)
αi,j (u, v) = m n
s=0
t=0 ws,t Ns,p (u)Nt,q (v)
(2)
αi,j is a constant for each particular pair of (u, v). Hence,
if the resolution of the polygon model does not change during the deformation, we may precompute the deformation
coefficients and update the polygon model incrementally as
shown in Equation (1).
Unfortunately, all the methods mentioned above cannot handle deforming trimmed NURBS surfaces efficiently. Regardless of the high computational cost of the tessellation process, it has to be performed repeatedly in every frame as the
surfaces are deforming. On the other hand, if the trimming
curve is undergoing deformation, methods adopting the uvmonotone approach will even need to re-generate a new set
of uv-monotone patches before the tessellation process.
However, when a surface deforms, its curvature is likely
changed and we need to refine the resolution of the polygon model and to compute new deformation coefficients incrementally according to the change in the surface curvature. A NURBS surface is first converted into a set of Bézier
patches using knot insertion [2]. Each Bézier patch is then
subdivided into a polygon model, which is maintained in a
quadtree hierarchy, by applying the de Casteljau subdivision formula [3] to the Bernstein polynomials in both u and
v directions. We refer to this as the polygon hierarchy. For
example, in u, we have:
Recently, Kahlesz et al. [8] developed an adaptive tessellation method. It recursively subdivides a surface into a
quad-tree hierarchy according to some approximation criteria. If a leaf quad-tree node is found to be untrimmed, it
will be selected as an output polygon for the resulting polygon model. Otherwise, the node is further subdivided for
further processing or be tessellated by constrained delaunay
triangulation to generate the output polygons. This method
was further enhanced by building a seam graph structure on
the tessellated trimmed NURBS surfaces for multi- resolution modeling [6]. A seam graph structure is actually a
progressive mesh [7] like structure. It allows the application
to select an appropriate resolution of the tessellated polygon
model for rendering. However, the construction of the seam
graph structure is itself an expensive process. Hence, this
w
wir−1 r−1
P
(u) + u i+1
P r−1 (u)
wir i
wir i+1
r−1
Pir (u) = (1 − u)
(3)
r−1
(u) and r = 1, . . . , n,
where wir (u) = (1 − u)wir−1 (u) + uwi+1
T
i = 0, . . . , n − r. [ w P w ] are the homogeneous Bézier
i
i
i
points with Pi ∈ R3 , wi are the weights, and n is the degree
of the surface. The v direction has similar recursion.
The difference of Equation (3) before and after the deformation can be simplified to get a de Casteljau formula as
49
node region is trimmed out by the trimming curve as shown
in figures 1(a) and 1(b), respectively. The former represents
a convex trimmed node and can be tessellated by a simple
triangulation algorithm. The latter may represent either a
monotonic chain trimmed node or a complex trimmed node.
follows:
αri (u) = (1 − u)αr−1
(u) + uαr−1
i
i+1 (u)
(4)
for r = 1, . . . , n, i = 0, . . . , n−r. Equation (4) indicates that
the deformation coefficients can be generated incrementally
by the de Casteljau subdivision formula. Hence, if the resolution of the polygon model needs to be increased, the new
deformation coefficients can be calculated from adjacent deformation coefficients stored at existing vertices using the
de Casteljau formula. To achieve a better performance, we
implemented this based on the Horner’s formula, of average
complexity O(n) as opposed to O(n2 ) when based on the de
Casteljau’s formula.
3.2 Handling of the Trimming Loops
A trimmed NURBS surface is defined by a set of trimming
loops together with the NURBS surface itself. Each trimming loop consists of a set of NURBS curves, which are
defined over the parametric space of the NURBS surface. A
NURBS curve C(t) may be defined as:
C(t) =
n
βi (t)Pi
(a)
(b)
Figure 1: Note trimming: (a) exterior node region is trimmed out, and (b) interior node region
is trimmed out.
(5)
i=0
where Pi denotes the control points. βi (t), similar to the
case of NURBS surfaces, represents the set of deformation
coefficients of the NURBS curve. It is defined as:
wi Ni,p (t)
βi (t) = m
s=0 ws Ns,p (t)
4.2 Monotonic Chain Trimmed Nodes
A polyline segment is a monotonic chain with respect to
axis L if the polylines of the inscribed trimming curve segment have at most 2 intersections to any L perpendicular
to L. It is similar to the monotone definition but a monotone refers to a polygon while a monotonic chain refers to
the polylines. In our method, the monotonic chain is respected to the u and the v axes in the parametric space as
uv-monotonic polylines. A monotonic chain trimmed node
is then a combination of the uv-monotonic polylines and the
corners of the trimmed node as shown in figure 2. By tessellating these monotonic regions as a whole, we can both
reduce the number of node subdivisions and minimize the
number of resulting polygons. As shown in figure 2(a), such
case exists when there are u-monotonic and v-monotonic
polylines inscribed in a node, in which the monotonic polylines are connected at their maxima, umax and vmax , and
minima, umin and vmin , as shown in figure 2(b). In other
words, there are four uv-monotonic polylines in the node,
which have the following properties:
(6)
To trim a NURBS surface, we subdivide the NURBS curves
against the polygon hierarchy of the NURBS surface to form
polylines. By comparing the intersection points between the
polylines and the polygon hierarchy in the parametric space,
we may obtain a list of polylines for each quadtree node in
the polygon hierarchy that are inscribed in the node.
4.
TESSELLATION OF TRIMMED NODES
To render a trimmed NURBS surface, according to the current viewing parameters, we select the appropriate quadtree
nodes in the polygon hierarchy and tessellate them based
on their types. We classify all the nodes into three types:
visible non-trimmed nodes, invisible non-trimmed nodes and
trimmed nodes. For a visible non-trimmed node, we just
need to split it into 2 triangles along its diagonal. For an
invisible non-trimmed node, as it is completely trimmed out
by some trimming loop, we would just ignore it and would
not render it. For a trimmed node, we further classify it
into one of the three types: convex trimmed node, monotonic chain trimmed node and complex trimmed node. They
are described in the following subsections.
v max
M upper-left
M upper-right
L1
u min
A convex trimmed node is a node with a convex trimmed
region. To classify such a node, we make use of the strong
convex hull property of the NURBS curve definition. We
perform an angle test on the control points of the curve
segment to verify the convexity of the trimmed region in the
node. Once such a node is identified, there are two possible
cases as shown in Figure 1. Either the exterior or the interior
L3
L4
M lower-left
v min
4.1 Convex Trimmed Nodes
L2
M lower-right
u max
(a)
(b)
Figure 2: Handling of a monotonic chain trimmed
node: (a) determining maxima and minima, and (b)
partitioning of the node into four regions according
to the maxima and minima.
50
In u direction:
determinant of the 3 vertices is calculated as follows:
ui − ulef t
(∆x + ui ) − ulef t vi − vupper (∆y + vi ) − vupper ∀x{x : (umin ≤ ux ≤ ux+1 ≤ umax ) ∧ (ux , ux+1 ) ∈ Lupper }
∀x{x : (umax ≥ ux ≥ ux+1 ≥ umin ) ∧ (ux , ux+1 ) ∈ Llower }
=
In v direction:
=
∀y{y : (vmin ≤ vy ≤ vy+1 ≤ vmax ) ∧ (vy , vy+1 ) ∈ Llef t }
[∆y(ui − ulef t ) + (ui − ulef t )(vi − vupper )] −
[∆x(vi − vupper ) + (ui − ulef t )(vi − vupper )]
∆y(ui − ulef t ) − ∆x(vi − vupper )
(7)
As ulef t ≤ umin and ∆y ≥ 0, the first part, ∆y(ui − ulef t ),
of equation 7 must be positive. In addition, as vupper ≥ vmin
and ∆x ≥ 0, the second part, −∆x(vi − vupper ), of equation
7 must also be positive. Therefore, the result of equation 7
must always be positive.
∀y{y : (vmax ≥ vy ≥ vy+1 ≥ vmin ) ∧ (vy , vy+1 ) ∈ Lright }
where Lupper and Llower denote the upper uv-monotonic
polylines (L1 ∪ L2 ) and the lower uv-monotonic polylines
(L3 ∪ L4 ), respectively. These polylines are partitioned by
{umax , umin }. Llef t and Lright denote the left uv-monotonic
polylines (L1 ∪ L4 ) and the right uv-monotonic polylines
(L2 ∪ L3 ), respectively. These polylines are partitioned by
{vmax , vmin }.
By combining the 4 uv-monotonic regions together, the result of the tessellation process may become as shown in figure 4. Since this tessellation process requires to identify
the 2 pairs of maxima and minima, the complexity is O(n)
bounded. In addition, the process traverses each vertex of
the polylines at most once, which is also O(n) bounded.
However, if a node is identified as a non-monotonic chain
trimmed node, i.e., it is a complex trimmed node, a further
subdivision is required.
All four uv-monotonic regions share the same properties, except that they have different orientations. To show how we
tessellate the node, we consider the upper-left uv-monotonic
region, Mupper−lef t , as shown in figure 3. Since a uv-monotonic region is convex in nature, a simplest way to tessellate it is by joining each vertex of the monotonic polylines
within the region to the nearest corner point of the region.
As an example, the nearest corner point of Mupper−lef t as
shown in figure 3 is Pupper−lef t , which has the coordinate
(ulef t , vupper ). Since the uv-monotonic polylines are inscribed in the node, we can have u lef t ≤ umin and vupper ≥
vmin . The tessellation is done by adding lines from corner point Pupper−lef t to each vertex Pi on the uv-monotonic
polylines of Mupper−lef t .
P upper-left(u left,v upper)
Figure 4: The resultant tessellation of a monotonic
chain trimmed node.
x
P i+1(u i+1,v i+1)
4.3 Complex Trimmed Nodes
When a node is identified as neither a convex trimmed node
nor a monotonic chain trimmed node, it is considered as
a complex trimmed node. Normally, if a node contains a
highly irregular trimming curve, it is most likely a complex
trimmed node. To handle this kind of nodes, we need to
further subdivide each of them into child nodes. This may
likely also involve subdividing the trimming curve to individual child nodes. Hence, this subdivision process essentially
reduces the irregularity of the trimming curve and allows
more and more subnodes to be classified as convex trimmed
nodes or monotonic chain trimmed nodes for triangulation.
On average, each child node contains about one-fourth of
the original trimming curve. Such a reduction in irregularity is very effective. According to our experiments, more
than 80% of nodes in a surface would be subdivided into
convex trimmed nodes or monotonic chain trimmed nodes.
Only 20% of the nodes need to be further subdivided. From
our experience, most nodes require only 1 or 2 levels of subdivision to partition a complex trimmed node into convex
trimmed nodes and/or monotonic chain trimmed nodes.
y
P i(u i,v i)
Figure 3: Tessellation of the upper-left uv-monotonic region.
To show that this tessellation process would work correctly,
i.e., the lines added would not cross each other, we consider
two consecutive sample points of a trimming curve segment,
i.e., the two end points of a polyline. Refer to figure 3 as
an example. The two end points are Pi and Pi+1 , with
Pi+1 being the next point of Pi . The two points form a
triangle with Pupper−lef t and the coordinates of the triangle
are (ulef t , vupper ), (ui , vi ) and (ui+1 , vi+1 ). If the orientation
of these 3 vertices is always turning left, their determinant
should then be positive. To evaluate the determinant, we
re-express Pi+1 as (∆x + ui , ∆y + vi ), where (∆x, ∆y) is the
offset from Pi to Pi+1 . According to the monotone property,
∆x and ∆y will always be positive in Mupper−lef t . The
51
5.
to decrease the resolution of C. By observation, an update
to C(t 3 ) will affect the polylines between C(t1 ) to C(t5 ). In
fact, C(t1 ) and C(t5 ) are the previous vertex and the next
vertex to C(t3 ), respectively. Hence, C(t1 ) and C(t5 ) may
be considered as the updated range of C(t 3 ). Generally
speaking, whether a resolution refinement process involves
inserting or deleting vertices, the update range always falls
between (max{t : t < ti }, ti ) and (ti , min{t : ti < t}).
OBJECT DEFORMATION
One of the critical developments in our research is that our
method supports real-time deformation of both the trimming curves and the trimmed NURBS surfaces. We note
that the deformation of a trimming curve does not affect
the topology or the shape of the trimmed NURBS surface.
On the other hand, the deformation of the trimmed NURBS
surface will only affect the shape of the surface itself; it does
not affect the shape of the trimming curve. As such, both
types of deformation are only loosely related to each other,
and we can handle each of them separately.
C(t 2)
5.1 Trimming Curve Deformation
C(t 3)
C(t 4)
C(t 5)
C(t 1)
Usually, when a trimming curve is being deformed, only part
of the NURBS surface is affected. It will be expensive to
perform the retriangulation of the trimming curve against
the polygon hierarchy of the NURBS surface in every frame
while the trimming curve is deforming. According to the
local modification property of the NURBS curves, any deformation driven by moving a control point Pi of a NURBS
curve will only affect the curve segment within the parametric range [ti , ti+p ), where p is the order of the trimming
curve. Hence, we may limit the update operation to within
this trimming curve segment defined by ti and ti+p . To allow an efficient update of the corresponding polylines of the
curve segment, we may simply check each of the polylines
to see if its two end points satisfy the following condition:
C(t 1)
C(t 2)
C(t 3)
C(t 5)
C(t 1)
C(t 1)
C(t 5)
C(t 5)
C(t 3)
(ti ≤ tstart ∧ tend ≤ ti+p )
C(t 4)
C(t 3)
C(t 2) C(t 4)
Inserting Child Nodes
C(t 2) C(t 4)
Deleting Child Nodes
(a)
(b)
Figure 5: Resolution refinement of a trimming curve
segment: (a) resolution increase, and (b) resolution
decrease.
Only those polylines satisfying this condition are affected
by the deformation and hence need to be updated. Once
we have identified the affected polylines, we would update
the parametric positions of their vertices. We then perform
resolution refinement and compute the new sets of deformation coefficients for the newly inserted vertices. The updated polylines are then remapped to the NURBS surface
for retessellation.
Note that the curvature change of the trimming curve segment due to the deformation of the NURBS surface is usually very small. Hence, only a very small number of vertices
may need to be inserted or deleted from the polylines in
order to maintain a good polygonal representation of the
trimmed surface. Figure 6 shows a trimmed NURBS surface before and after deformation. We can see that both
the resolution of the surface itself and the resolution of the
trimming curve are refined according to the change of the
surface curvature.
5.2 Trimmed Surface Deformation
When a trimmed NURBS surface deforms, the curvature of
the surface may be affected and the curvature of the trimming curves within the deformed region of the surface may
also be affected. We handle this in a way somewhat similar to how we handle the deformation of the trimming curve.
However, the scope of the update is relatively smaller. First,
we update the affected region of the NURBS surface. Second, we update the vertex positions of the affected polylines
on the NURBS surface. Third, we perform resolution refinement on the affected trimming curve segment. Finally, we
retessellate the resulting polylines with the corresponding
nodes.
Figure 5 illustrates two examples of resolution refinement
of a trimming curve segment C with reference to a node in
the polygon hierarchy, where C(ti ) denotes a vertex of the
polylines representing C. In figure 5(a), we assume that the
polylines can no longer approximate C due to the increase
in the surface curvature. Hence, we need to increase the
resolution of C by inserting vertices C(t2 ) and C(t4 ) to the
polylines. On the other hand, if the resolution of C is found
to be too high due to the decrease in the surface curvature as
shown in figure 5(b), we may delete vertices C(t2 ) and C(t4 )
(a)
(b)
Figure 6: Resolution refinement of a trimmed
NURBS surface: (a) before deformation, and (b)
after deformation.
52
Time (Sec)
Time (Sec)
Trimming Curve Deformation Only
0.120
0.120
0.100
0.100
0.080
0.080
0.060
0.060
0.040
0.040
OpenGL
Our Methods
0.020
0.000
0
100
200
300
Frame Number
Surface Deformation Only
OpenGL
Our Methods
0.020
0.000
400
500
0
100
200
300
Frame Number
400
500
Figure 7: Performance comparison between our method and the method used by OpenGL.
Time (Sec)
Time (Sec)
Trimming Curve Deformation Only
0.050
Surface Deformation Only
0.050
Rendering
Coordinate Update
Tessellation
0.040
Rendering
Coordinate Update
Tessellation
0.040
0.030
0.030
0.020
0.020
0.010
0.010
0.000
0.000
0
100
200
300
Frame Number
400
500
0
100
200
300
Frame Number
400
500
Figure 8: Computational costs of individual operations.
6.
RESULTS AND DISCUSSIONS
If we compare trimming curve deformation with trimmed
surface deformation as shown in figure 7, we can see that
the performance of trimming curve deformation is relatively
fluctuating. This is because whenever a trimming curve
is deformed, we need to perform the retessellation process.
The efficiency of this retessellation process is mainly affected
by the curvature of the trimming curve, since a smoother
trimming curve will generate less polylines. For the randomly deforming trimming curves, their curvatures are expected to vary significantly. As a result, the computation
time would be unstable. In contrast, the surface deformation does not generate a massive update of trimmed surface
patches and hence, the computation time is relatively stable.
We have implemented the new method in C++ and OpenGL.
All the experiments presented here have been performed on
a Pentium 4 2.0GHz machine with 256 MB RAM. The deformation events were triggered by randomly moving either
40% of the trimming curve control points, or 5% of the surface control points in each frame. Figure 9 shows a human face model that we used to test the proposed rendering
method. The model is constructed by a single NURBS surface containing 837 control points and 9 separate trimming
curves. The figure shows the model before and after deformation, in shaded and wireframe rendering. Figure 10 shows
another test model of a classical teapot after deformation.
Figure 8 shows the performance of individual operations
used in our method. Tessellation refers to the classification and triangulation of all the trimmed nodes. Coordinate
update refers to the incremental updating of the trimming
curves and the trimmed surface. Rendering refers to the
time taken to render all the polygons by the OpenGL engine
in each frame. We can see that the tessellation process only
needs to be executed occasionally in the surface deformation
case. For trimming curve deformation, as the surface coordinates of the polylines cannot be calculated with the deformation coefficients, they should be recomputed from the
coordinates of the incrementally updated trimming curve.
Hence, this process is in general more computationally intensive for trimming curve deformation than for trimmed
To demonstrate the performance of our method, we have
compared it with the method used by OpenGL, i.e., the
variant of Rockwood’s method [17], using the human face
model shown in figure 9. Figure 7 shows the rendering times
of the two methods, we can see that our method in each
consecutive frame is roughly 2.5 and 3 times faster than
the OpenGL method for trimming curve deformation and
for trimmed surface deformation, respectively. The reason
for this is that as we deform the trimmed surface and the
trimming curves continuously, a complete evaluation is required in each frame for the OpenGL method. However, our
method only needs to perform an incremental update to the
affected region and is thus more efficient.
53
[9] S. Kumar, D. Manocha, and A. Lastra. Interactive
Display of Large-Scale NURBS Models. In Proc. of
Symposium on Interactive 3D Graphics, pages 51–58,
1995.
surface deformation. When a trimming curve deforms, we
need to perform re-evaluation on the curve to generate the
updated polylines representing the deformed curve. However, in practice, this process may not significantly degrade
the overall rendering performance as the region of the deformation, and hence, the effort spent on the update, is usually
relatively small compared with the deformation of the whole
surface.
7.
[10] S. Kumar, D. Manocha, H. Zhang, and K. Hoff.
Accelerated Walkthrough of Large Spline Models. In
Proc. of Symposium on Interactive 3D Graphics, pages
91–101, 1997.
[11] F. Li and R. Lau. Incremental Polygonization of
Deforming NURBS Surfaces. Journal of Graphics
Tools, 4(4):37–50, 1999.
CONCLUSION
In this paper, we have introduced an interactive rendering
method for deformable trimmed NURBS surfaces. In order
to efficiently update the trimmed NURBS surface and the
trimming curves as the surface and/or the trimming curves
are deforming, we propose a regional update mechanism and
an efficient method for dynamically tessellating the NURBS
surface with the trimming curves. We have shown that the
new method for rendering deformable trimmed NURBS surfaces is more efficient than the method used in OpenGL during the curve/surface deformation.
8.
[12] F. Li and R. Lau. Real-Time Rendering of Deformable
Parametric Free-Form Surfaces. In Proc. of ACM
VRST, pages 131–138, December 1999.
[13] F. Li, R. Lau, and M. Green. Interactive Rendering of
Deforming NURBS Surfaces. In Proc. of Eurographics
’97, pages 47–56, September 1997.
[14] F. Li, R. Lau, and F. Ng. Collaborative Distributed
Virtual Sculpting. In Proc. of of IEEE VR’01, pages
217–224, March 2001.
ACKNOWLEDGMENTS
[15] F. Li, R. Lau, and F. Ng. VSculpt: A Distributed
Virtual Sculpting Environment for Collaborative
Design. IEEE Trans. on Multimedia (to appear), 2003.
The work described in this paper was partially supported by
two SRG grants (Project Numbers: 7001391 and 7001465)
and a DAG grant (Project Number: 7100264), all from City
University of Hong Kong.
9.
[16] L. Piegl and W. Tiller. The NURBS Book Second
edition. Springer-Verlag, Tyler, TX, 1997.
[17] A. Rockwood, K. Heaton, and T. Davis. Real-Time
Rendering of Trimmed Surfaces. In Proc. of ACM
SIGGRAPH ’89, volume 23, pages 107–116, 1989.
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54
Figure 9: A human face modeled by a trimmed NURBS surface: (upper) shaded and wireframe before
deformation, and (lower) shaded and wireframe after deformation on the eyes, mouth and face.
Figure 10: A teapot modeled by a trimmed NURBS surface: shaded and wireframe before deformation.
55
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